Comparisons of Quantitative Parameters of Ga-68-Labelled Fibroblast Activating Protein Inhibitor (FAPI) PET/CT and [18F]F-FDG PET/CT in Patients with Liver Malignancies

Role of molecular imaging in prognosis, diagnosis, and treatment of gastrointestinal cancers: An update on new therapeutic methods

One of the leading causes of cancer-related death is gastrointestinal cancer, which has a significant morbidity and mortality rate. Although preoperative risk assessment is essential for directing patient care, its biological behavior cannot be accurately predicted by conventional imaging investigations. Potential pathophysiological information in anatomical imaging that cannot be visually identified can now be converted into high-dimensional quantitative image features thanks to the developing discipline of molecular imaging. In order to enable molecular tissue profile in vivo, molecular imaging has most recently been utilized to phenotype the expression of single receptors and targets of biological therapy. It is expected that molecular imaging will become increasingly important in the near future, driven by the expanding range of biological therapies for cancer. With this live molecular fingerprinting, molecular imaging can be utilized to drive expression-tailored customized therapy. The technical aspects of molecular imaging are first briefly discussed in this review, followed by an examination of the most recent research on the diagnosis, prognosis, and potential future clinical methods of molecular imaging for GI tract malignancies.

Diagnostic Performances of PET/CT Using Fibroblast Activation Protein Inhibitors in Patients with Primary and Metastatic Liver Tumors: A Comprehensive Literature Review

PET/CT using radiolabeled fibroblast activation protein inhibitors (FAPIs) is a promising diagnostic tool in oncology, especially when non-increased and/or physiologically high [18F]FDG uptake (as in liver parenchyma) is observed. We aimed to review the role of PET/CT using radiolabeled FAPIs in primary and/or metastatic liver lesions, and to compare their performances with more "conventional" radiopharmaceuticals. A search algorithm based on the terms "FAPI" AND ("hepatic" OR "liver") was applied, with the last update on 1st January 2024. Out of 177 articles retrieved, 76 studies reporting on the diagnostic application of radiolabeled FAPI PET/CT in at least one patient harboring primary or metastatic liver lesion(s) were fully analyzed. Although there was some heterogeneity in clinical conditions and/or study methodology, PET/CT with radiolabeled FAPIs showed an excellent performance in common primary liver malignancies (hepatocarcinoma, intrahepatic cholangiocarcinoma) and liver metastases (mostly from the gastrointestinal tract and lungs). A higher tumor-to-background ratio for FAPIs than for [18F]FDG was found in primary and metastatic liver lesions, due to lower background activity. Despite limited clinical evidence, radiolabeled FAPIs may be used to assess the suitability and effectiveness of FAPI-derived therapeutic agents such as [177Lu]Lu-FAPI. However, future prospective research on a wider population is needed to confirm the excellent performance.

Diagnostic and Therapeutic Application of Fibroblast Activation Protein Inhibitors in Oncologic and Nononcologic Diseases

ABSTRACT

Fibroblast activation protein inhibitor positron emission tomography (PET) has gained interest for its ability to demonstrate uptake in a diverse range of tumors. Its molecular target, fibroblast activation protein, is expressed in cancer-associated fibroblasts, a major cell type in tumor microenvironment that surrounds various types of cancers. Although existing literature on FAPI PET is largely from single-center studies and case reports, initial findings show promise for some cancer types demonstrating improved imaging when compared with the widely used 18F-fludeoxyglucose PET for oncologic imaging. As we expand our knowledge of the utility of FAPI PET, accurate understanding of noncancerous uptake seen on FAPI PET is crucial for accurate evaluation. In this review, we summarize potential diagnostic and therapeutic applications of radiolabeled FAP inhibitors in oncological and nononcological disease processes.

[68Ga]Ga-FAPI-46 synthesis on a GAIA® module system: Thorough study of the automated radiolabeling reaction conditions

The influence of several parameters involved in the 68Ga radiolabeling of FAPI-46 was studied at the scale of the automated reaction. Among the buffers tested, HEPES 0.3 M pH 4 allowed both high radiochemical purity (RCP) and radiochemical yield (RCY), without prepurification of 68Ga but after final purification of [68Ga]Ga-FAPI-46 on a C18 cartridge. A longer reaction time did not show significant benefit on the RCP, while higher loads of FAPI-46 and gentisic acid as anti-radiolysis compound allowed better RCY.

PET/CT FAPI: Procedure and evidence review in oncology

Neoplasms are composed of malignant tumor cells, which are surrounded by other non-tumor cellular elements, in what has been defined as the microenvironment or tumor stroma. Evidence on the importance of the tumor microenvironment has not stopped growing in recent years. It plays a central role in cell proliferation, tissue invasion, angiogenesis and cell migration. The paradigm is the family of new FAPI radiopharmaceuticals that show the density of the fibroblast activation protein (FAP) which is overexpressed in the cell membrane of activated cancer-associated fibroblasts (CAF), and its presence is related to poor prognosis. This educational document includes the procedure for performing PET/CT FAPI, biodistribution and the main potentially clinical applications in oncology to date.

The inferior performance of [68Ga]Ga-FAPI-04 PET/CT as a diagnostic and theranostic biomarker in [177Lu]Lu-DOTATATE refractory well-differentiated neuroendocrine tumors

PURPOSE

We aimed to investigate the potential of [68Ga]Ga-FAPI-04 PET/CT as an alternative diagnostic and theranostic tool in well-differentiated NETs refractory to [177Lu]Lu-DOTATATE therapy.

METHODS

Patients who received at least two cycles of [177Lu]Lu-DOTATATE therapy for metastatic NETs and progressed under treatment were included. All patients had performed [68Ga]Ga-DOTATATE and [68Ga]Ga-FAPI-04 PET/CT within 3 weeks. The number of PET-positive lesions related to NETs and tumor sites was documented. Mann-Whitney U and chi-square tests were used to compare SUVmax levels of tracers and the number of detected metastases.

RESULTS

Twelve patients (7 male, 5 female) who met the eligibility criteria were included in the study. Ten patients had grade 1-2 NET of various origins, and two had paraganglioma and pheochromocytoma. One hundred ninety-eight of 230 lesions (86%) were SSTR positive with a median SUVmax of 16.6 (2.2-76.5), and 88 of 230 lesions (38.2%) were [68Ga]Ga-FAPI-04 positive with a median SUVmax of 5.1 (2.3-21). Median SUVmax level and detected number of tumors were significantly higher in [68Ga]Ga-DOTATATE PET/CT (p=<0.001). [68Ga]Ga-FAPI-04 PET/CT was completely (n:2) or almost completely (n:3) negative in 5 (42%) patients. Two (17%) patients had flip-flop SSTR/FAPI uptake in tumors. In four patients (33%), tumor uptake or the number of PET-positive lesions was inferior in [68Ga]Ga-FAPI-04 PET/CT. In only one patient (8%), tumor uptakes were higher in [68Ga]Ga-FAPI-04 PET/CT. Low-dose [177Lu]Lu-FAPI-46 dosimetry was performed on the FAPI-dominant patient; absorbed radiation doses per GBq were 1.26 Gy, 0.36 Gy, 0.32 Gy, and 0.2 Gy for kidneys, liver, spleen, and total body, respectively. The mean absorbed dose per GBq was 0.33 Gy for liver mass and 0.41 Gy for metastatic lymph nodes.

CONCLUSION

Our preliminary results demonstrated that [68Ga]Ga-FAPI-04 PET/CT mainly failed in well-differentiated NETs refractory to [177Lu]Lu-DOTATATE therapy and had a limited role as an alternative diagnostic or theranostic agent. Further investigations with a larger patient population are required to determine the impact of [68Ga]Ga-FAPI-04 PET/CT on NETs.

Head-to-head comparison of 18F-FAPI and 18F-FDG PET/CT in staging and therapeutic management of hepatocellular carcinoma

BACKGROUND

Fluorine 18 (18F) fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) has limitations in staging hepatocellular carcinoma (HCC). The recently introduced 18F-labeled fibroblast-activation protein inhibitor (FAPI) has shown promising prospects in detection of HCC lesions. This study aimed to investigate the initial staging and restaging performance of 18F-FAPI PET/CT compared to 18F-FDG PET/CT in HCC.

METHODS

This prospective study enrolled histologically confirmed HCC patients from March 2021 to September 2022. All patients were examined with 18F-FDG PET/CT and 18F-FAPI PET/CT within 1 week. The maximum standard uptake value (SUVmax), tumor-to-background ratio (TBR), and diagnostic accuracy were compared between the two modalities.

RESULTS

A total of 67 patients (57 men; median age, 57 [range, 32-83] years old) were included. 18F-FAPI PET showed higher SUVmax and TBR values than 18F-FDG PET in the intrahepatic lesions (SUVmax: 6.7 vs. 4.3, P < 0.0001; TBR: 3.9 vs. 1.7, P < 0.0001). In diagnostic performance, 18F-FAPI PET/CT had higher detection rate than 18F-FDG PET/CT in intrahepatic lesions [92.2% (238/258) vs 41.1% (106/258), P < 0.0001] and lymph node metastases [97.9% (126/129) vs 89.1% (115/129), P = 0.01], comparable in distant metastases [63.6% (42/66) vs 69.7% (46/66), P > 0.05]. 18F-FAPI PET/CT detected primary tumors in 16 patients with negative 18F-FDG, upgraded T-stages in 12 patients and identified 4 true positive findings for local recurrence than 18F-FDG PET, leading to planning therapy changes in 47.8% (32/67) of patients.

CONCLUSIONS

18F-FAPI PET/CT identified more primary lesions, lymph node metastases than 18F-FDG PET/CT in HCC, which is helpful to improve the clinical management of HCC patients.

TRIAL REGISTRATION

Clinical Trials, NCT05485792 . Registered 1 August 2022, Retrospectively registered.

Head-to-Head Comparison of FDG and Radiolabeled FAPI PET: A Systematic Review of the Literature

FAPI-based radiopharmaceuticals are a novel class of tracers, mainly used for PET imaging, which have demonstrated several advantages over [18F]FDG, especially in the case of low-grade or well-differentiated tumors. We conducted this systematic review to evaluate all the studies where a head-to-head comparison had been performed to explore the potential utility of FAPI tracers in clinical practice. FAPI-based radiopharmaceuticals have shown promising results globally, in particular in detecting peritoneal carcinomatosis, but studies with wider populations are needed to better understand all the advantages of these new radiopharmaceuticals.

FAP-targeted PET imaging in gastrointestinal malignancies: a comprehensive review

F18-fluorodeoxyglucose positron emission tomography/computed tomography (FDG PET/CT) plays a crucial role in tumour diagnosis, staging, and therapy response evaluation of various cancer types and has been a standard imaging modality used in clinical oncology practice for many years. However, it has certain limitations in evaluating some particular gastrointestinal cancer types due to low FDG-avidity or interphering physiological background activity. Fibroblast activation protein (FAP), a protein of the tumour microenvironment, is overexpressed in a wide range of cancers which makes it an attractive target for both tumour imaging and therapy. Recently, FAP-targeted radiopharmaceuticals are widely used in clinical research and achieved great results in tumour imaging. Considering the limitations of FDG PET/CT and the lack of physiological FAP-targeted tracer uptake in liver and intestinal loops, gastrointestinal cancers are among the most promising indications of FAP-targeted imaging. Herein, we present a comprehensive review of FAP-targeted imaging in gastrointestinal cancers in order to clarify the current and potential future role of this class of molecules in gastrointestinal oncology.

Head-to-head comparison of 68Ga-FAPI-04 PET/CT and 18F-FDG PET/CT in the evaluation of primary digestive system cancer: a systematic review and meta-analysis

Introduction

Althoug ¹⁸F-FDG positron emission tomography/computed tomography (PET/CT) is widely accepted as a diagnostic tool for detecting digestive cancers, ⁶⁸Ga-FAPI-04 PET/CT may perform better in detecting gastrointestinal malignancies at an earlier stage. This study aimed to systematically review the diagnostic performance of ⁶⁸Ga-FAPI-04 PET/CT compared with that of ¹⁸F-FDG PET/CT in primary digestive system cancers.

Methods

In this study, a comprehensive search using the PubMed, EMBASE, and Web of Science databases was performed to identify studies that met the eligibility criteria from the beginning of the databases to March 2023. The quality of the relevant studies with the Quality Assessment of Diagnostic Accuracy Studies (QUADAS-2) method was assessed using the RevMan 5.3 software. Sensitivity and specificity were calculated using bivariate random-effects models, and heterogeneity was assessed with the I² statistic and meta-regression analysis using the R 4.22 software.

Results

A total of 800 publications were identified in the initial search. Finally, 15 studies comprising 383 patients were included in the analysis. The pooled sensitivity and specificity of ⁶⁸Ga-FAPI-04 PET/CT were 0.98 (95% CI, 0.94-1.00) and 0.81 (95% CI, 0.23-1.00), whereas those of ¹⁸F-FDG PET/CT were 0.73 (95% CI, 0.60-0.84) and 0.77 (95% CI, 0.52-0.95), respectively. ⁶⁸Ga-FAPI-04 PET/CT performed better for specific tumours, particularly in gastric, liver, biliary tract, and pancreatic cancers. Both imaging modalities had essentially the same diagnostic efficacy in colorectal cancer.

Conclusions

⁶⁸Ga-FAPI-04 PET/CT showed a higher diagnostic ability than ¹⁸F-FDG PET/CT in terms of diagnosing primary digestive tract cancers, especially gastric, liver, biliary tract, and pancreatic cancers. The certainty of the evidence was high due to the moderately low risk of bias and low concern regarding applicability. However, the sample size of the included studies was small and heterogeneous. More high-quality prospective studies are needed to obtain higher-quality evidence in the future.

Systematic Review Registration

The systematic review was registered in PROSPERO [CRD42023402892].

Evaluation of the Diagnostic Accuracy of FAPI PET/CT in Oncologic Studies: Systematic Review and Metaanalysis

Fibroblast-activation protein is a promising target for oncologic molecular imaging. Studies show that fibroblast activation protein inhibitor (FAPI) radiotracers are accurate diagnostics with favorable tumor-to-background ratios across various cancers. Therefore, we performed a systematic review and metaanalysis to assess the diagnostic performance of FAPI PET/CT in comparison with [¹⁸F]FDG PET/CT, the most widely used radiotracer in oncology. Methods: We conducted a systematic search in MEDLINE, Embase, Scopus, PubMed, Cochrane Central Register of Controlled Trials, relevant trial registries, and bibliographies. The search consisted of combinations of terms for 3 topics: neoplasia, PET/CT, and FAPI. Two authors independently screened retrieved articles using predefined inclusion and exclusion criteria and extracted the data. Study quality was assessed using the criteria of QUADAS-2 (Quality Assessment of Diagnostic Accuracy Studies 2). For each study, the sensitivity, specificity, and 95% CIs were calculated to determine diagnostic accuracy for primary, nodal, and metastatic lesions. A random-effects metaanalysis was used for pooling the data, and heterogeneity was assessed (I² index). Results: Thirty-nine studies (1,259 patients) investigating the use of FAPI PET/CT were included. On a patient-based analysis, pooled sensitivity was 0.99 (95% CI, 0.97-1.0) for the detection of primary lesions. Pooled sensitivity for nodal and distant metastases was 0.91 (95% CI, 0.81-0.96) and 0.99 (95% CI, 0.96-1.0), respectively. On a paired analysis between FAPI and [¹⁸F]FDG PET/CT, FAPI had a higher sensitivity in the detection of primary, nodal, and metastatic lesions (all P < 0.001). The differences in sensitivities between FAPI and [¹⁸F]FDG were statistically significant. In terms of heterogeneity, analyses on primary lesions were moderately affected, distant metastatic lesions were highly affected, and the nodal metastatic analyses had negligible heterogeneity. Conclusion: The diagnostic performance of FAPI PET/CT is superior to that of [¹⁸F]FDG in the detection of primary, nodal, and distant metastases. However, further studies are needed to better evaluate its utility and indication in specific cancer types and clinical settings.

The Diagnostic Value of Fibroblast Activation Protein Imaging in Hepatocellular Carcinoma and Cholangiocellular Carcinoma

Hepatocellular carcinoma (HCC) is the most common primary liver malignancy with worldwide high incidence and mortality. In more than 90% of cases, HCC arise from a cirrhotic liver that is mostly induced by viral diseases and especially in developed countries alcoholic steatohepatitis and non-alcoholic steatohepatitis. In contrast, cholangiocellular carcinoma (CCC) is a very rare cancer entity with a high mortality due to insidious onset. The only curative option for both cancer entities is a timely and definitive surgical therapy, which mandates an accurate early diagnosis. To this end, [18F]FDG PET/CT scan could demonstrate only little benefit, as there is an unmet clinical need for an alternative, pan-cancer agent for initial diagnostic work-up of CCC or evaluation of Milan criteria for HCC patients.

Update on PET Radiopharmaceuticals for Imaging Hepatocellular Carcinoma

Numerous positron emission tomography (PET) targets for detection and staging of hepatocellular cancer have been developed in recent years. Hepatocellular carcinomas (HCCs) are clinically and pathologically heterogeneous tumours with a high tendency to be aggressive and unresponsive to chemotherapy. Early detection is essential, and the need for an adequate imaging biomarker, which can overcome some of the limitations of conventional radiological imaging, is persistent. Flourine-18 (18F) flourodeoxyglucose (FDG), the most widely used PET radiopharmaceutical, has proven disappointing as a possible staple in the evaluation of HCC. This disappointment had led to experimentation with carious radiotracers, such as the choline derivatives, acetate, and prostate-specific membrane antigen, which appear to complement and/or enhance the role of FDG. In this study, we look at the various PET radiopharmaceuticals that have been used for imaging HCC and the particular pathways that they target in HCC and liver cancers.

Current research topics in FAPI theranostics: a bibliometric analysis

PURPOSE

The study aimed to provide a comprehensive bibliometric overview of the current scientific publications on fibroblast activation protein inhibitor (FAPI) positron emission tomography imaging and radionuclide therapy.

METHODS

A PubMed search was performed to identify all MEDLINE-indexed publications on FAPI imaging and radionuclide therapy. The last update was performed on 31 May 2022. An online database of this literature was created, and hierarchical topic-related tags were subsequently assigned to all relevant studies. Frequency analysis was used to evaluate the distribution of the following characteristics: first author's country of origin, journal of publication, study design, imaging techniques and radiopharmaceutical used, histopathological correlation, the type of cancer, and benign disease/uptake types evaluated.

RESULTS

A total of 294 relevant publications on original studies were identified, consisting of 209 (71%) case reports/series and 85 cohort studies (29%). The majority of studies focused on imaging topics, predominantly comparing uptake on FAPI-PET/CT with 2-[¹⁸F]FDG-PET/CT, anatomical imaging, and/or histopathology results. 68% of studies focused on malignancies, with gastro-intestinal cancer, hepato-pancreato-biliary cancer, mixed cancers/metastases, lung cancer, sarcoma, head and neck cancer, and breast cancer being the most frequently reported. 42% of studies focused on benign disease categories, with cardiovascular, musculoskeletal, HPB, head and neck, and IgG4-related disease as most common categories. 16/294 (5%) studies focused on radionuclide therapy, with preliminary reports of acceptable toxicity profiles, tumour activity retention, and suggestion of disease control.

CONCLUSION

FAPI research is rapidly expanding from diagnostic studies in malignancies and benign diseases to the first reports of salvage radionuclide therapy. The research activity needs to shift now from low-level-of-evidence case reports and series to prospectively designed studies in homogenous patient groups to provide evidence on how and in which clinical situations FAPI theranostics can be of added value to clinical care. We have provided an overview of current research topics to build upon.

The Superiority of Fibroblast Activation Protein Inhibitor (FAPI) PET/CT Versus FDG PET/CT in the Diagnosis of Various Malignancies

Cancer represents a major cause of death worldwide and is characterized by the uncontrolled proliferation of abnormal cells that escape immune regulation. It is now understood that cancer-associated fibroblasts (CAFs), which express specific fibroblast activation protein (FAP), are critical participants in tumor development and metastasis. Researchers have developed various FAP-targeted probes for imaging of different tumors from antibodies to boronic acid-based inhibitor molecules and determined that quinoline-based FAP inhibitors (FAPIs) are the most appropriate candidate as the radiopharmaceutical for FAPI PET/CT imaging. When applied clinically, FAPI PET/CT yielded satisfactory results. Over the past few years, the utility and effectiveness of tumor detection and staging of FAPI PET/CT have been compared with FDG PET/CT in various aspects, including standardized uptake values (SUVs), rate of absorbance and clearance. This review summarizes the development and clinical application of FAPI PET/CT, emphasizing the diagnosis and management of various tumor types and the future prospects of FAPI imaging.

Comparison of 68Ga-FAPI and 18F-FDG PET/CT for the diagnosis of primary and metastatic lesions in abdominal and pelvic malignancies: A systematic review and meta-analysis

Purpose

The purpose of this study is to compare the application value of ⁶⁸Ga-FAPI and ¹⁸F-FDG PET/CT in primary and metastatic lesions of abdominal and pelvic malignancies (APMs).

Materials

The search, limited to the earliest available date of indexing through 31 July 2022, was performed on PubMed, Embase, and Cochrane Library databases using a data-specific Boolean logic search strategy. We calculated the detection rate (DR) of ⁶⁸Ga-FAPI and ¹⁸F-FDG PET/CT in the primary staging and recurrence of APMs, and pooled sensitivities/specificities based on lymph nodes or distant metastases.

Results

We analyzed 473 patients and 2775 lesions in the 13 studies. The DRs of ⁶⁸Ga-FAPI and ¹⁸F-FDG PET/CT in evaluating the primary staging and recurrence of APMs were 0.98 (95% CI: 0.95-1.00), 0.76 (95% CI: 0.63-0.87), and 0.91(95% CI: 0.61-1.00), 0.56 (95% CI: 0.44-0.68), respectively. The DRs of ⁶⁸Ga-FAPI and ¹⁸F-FDG PET/CT in primary gastric cancer and liver cancer were 0.99 (95% CI: 0.96-1.00), 0.97 (95% CI: 0.89-1.00) and 0.82 (95% CI: 0.59-0.97), 0.80 (95% CI: 0.52-0.98), respectively. The pooled sensitivities of ⁶⁸Ga-FAPI and ¹⁸F-FDG PET/CT in lymph nodes or distant metastases were 0.717(95% CI: 0.698-0.735) and 0.525(95% CI: 0.505-0.546), and the pooled specificities were 0.891 (95% CI: 0.858-0.918) and 0.821(95% CI: 0.786-0.853), respectively.

Conclusions

This meta-analysis concluded that ⁶⁸Ga-FAPI and ¹⁸F-FDG PET/CT had a high overall diagnostic performance in detecting the primary staging and lymph nodes or distant metastases of APMs, but the detection ability of ⁶⁸Ga-FAPI was significantly higher than that of ¹⁸F-FDG. However, the ability of ⁶⁸Ga-FAPI to diagnose lymph node metastasis is not very satisfactory, and is significantly lower than that of distant metastasis.

Systematic review registration

https://www.crd.york.ac.uk/prospero/, identifier CRD42022332700.